What does Weft (also Filling) mean?
A construction in yarn in which the lengthwise, selvage to selvage horizontal is carried over and under the weft. In contrast to warp, this yarn has less twists and therefore less strain.
Heddels explains Weft (also Filling)
The weft, or filling, threads are the horizontal threads passed through the warp threads via the shuttle during the weaving process. The term weft comes from the Old English word “wefan”, meaning “to weave”. Because weft threads are subjected to far less stress and tension than warp threads, they are usually not as strong. With jeans, the weft threads are typically white or ecru and are only visible on the wrong side of the denim.